There is a 3.5mm jack in the back of the computer so you could put it on your desk or something. It's hard to tell from the question whether or not you want it to be mounted internally. If so, you could take out one of the PCI placeholder things, run the AUX Cord through the hold you just created, and plug it into the audio out port. On the inside of the case, you could probably manage to mount it somewhere. That's the best answer I have, you could also probably manage to splice a larger speaker into the existing cable
Generally, fitting any kind of external speaker or headphones will cause the internal one to stop working. The one exception to this I can think of is the Griffin "iSub" which is a USB subwoofer that was really designed to complement the built-in iMac speakers but will work with PowerPC Macs up to 10.4.11 officially and 10.5.8 unofficially.
Look in the applications folder, inside the utilities folder, theres is an application called Audio MIDI.
In there you can configure a profile in which multiple speaker outputs are used ate the same time.
I haven't messed with it in a while, but from what I can remember, you cannot do it via an AUX cable, you would need a bluetooth speaker...
Hope this helps
@charlieroberts Unfortunately i do not have a bluetooth speaker, but you gave me a good idea, and I found a wonderful app that makes what you said possible!!
Here is Airfoil from Roguea Amoeba, available for powerpc, it works with android phones, iphones and wifi speakers!! (the only problem is that after 10 minutes a noise make your music unbearable to listen, unless you pay for the app, but you can avoid this by restarting the app)